Even if they won't give you credit for what you paid for the flash drive, it would be worth paying another $20 to get a set of DVD factory recovery disks. I have found that doing a factory restore from DVD always works; but in my opinion, flash drives have some issues here and there, and flash drives sometimes go bad.
If you get a set of DVDs, make a couple of copies of the entire set, so you never have to order them again.
The Drivers and Utilities disk is just that -- it contains the drivers you need for your computer, plus some utilities. Unfortunately, it doesn't include Windows itself.
As an alternative, you could go to the following website and download their Windows and Office ISO download Tool:
Microsoft Windows and Office ISO Download Tool
You can download every currently supported version of Windows or Office with that tool. Create a DVD using the ISO file that you download, then install Windows from the DVD using your Windows 8.1 install key. I just did that with Windows 7 -- there was an OEM install key on a sticker on the computer, and I successfully activated Windows with that install key. If you do the install this way, you may need to install some Windows drivers afterward. I didn't -- amazingly everything worked, and all devices showed as error-free in Device Manager!
Once you get everything completely done, do a backup of your computer using something like Macrium Reflect Free and an external hard drive. And be sure to create a Macrium emergency boot disk, so that you can completely recover in case of a future disk crash.